Reword chunk references, LSB usually means 'bits'. (#364)

https://en.wikipedia.org/wiki/Bit_numbering#Least_significant_bit

Signed-off-by: Catalin Patulea <[email protected]>

Author: cpatulea

docs/format/chunks.md

@@ -2,7 +2,7 @@
 
 The following describes the format of a single chunks file, which is created in the `chunks/` directory of a block. The maximum size per segment file is 512MiB.
 
-Chunks in the files are referenced from the index by the in-file offset in the 4 LSB and the segment sequence number in the higher 4 MSBs.
+Chunks in the files are referenced from the index by uint64 composed of in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
 
 ```
 ┌────────────────────────────────────────┬──────────────────────┐